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VW tdi 90bhp '99 T reg down on power, black smoke


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I'm not certain, but I think your engine has a wastegated turbo rather than a variable nozzle turbine (VNT) turbo. What you're experiencing is "limp mode" where the ECU is shutting down the turbo boost system because it senses a problem somewhere. It could be the maf (Mass Air Flow) sensor reading incorrect air volume to the ecu, but it's more likely to be an overboost situation (or the reverse, [underboost?]) where the ecu isn't getting the required amount of boost from the turbo and shutting it down.

A common problem with TDI's is if they've been tootled around for many years and not given a good flogging, they coke up the inlet manifold. This has the effect of causing the turbo to try forcing air into a smaller intake tract, hence more boost pressure than there should be.

Maybe a blocked intercooler? Whip it off and clean it out, see what it's like.

I think your engine also has the N75 valve system. This controls the wastegate to give the required boost conditions, it's a solenoid vacuum control valve. If it's sticking, this'll lead to overboost problems.

Of all these, my favourite is the intake manifold. But it's just a guess based on mileage.

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